Fabrication of solar cells with electrically conductive polyimide adhesive

ABSTRACT

The present disclosure provides a method of manufacturing a solar cell including: providing a first substrate and a second substrate; depositing on the first substrate a sequence of layers of semiconductor material forming a solar cell including a top subcell and a bottom subcell; forming a back metal contact over the bottom subcell; applying a conductive polyimide adhesive to the second substrate; attaching the second substrate on top of the back metal contact; and removing the first substrate to expose the surface of the top subcell.

This is a divisional of patent application Ser. No. 13/961,354, filedAug. 7, 2013, (pending), which is incorporated herein by reference inits entirety.

REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.13/831,406, filed Mar. 14, 2013, which is herein incorporated byreference in its entirety.

GOVERNMENT RIGHTS STATEMENT

This invention was made with government support under Contract No.11-C-0585 awarded by the National Reconnaissance Office (NRO). TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor devices, andto fabrication processes and devices such as multijunction solar cellsbased on III-V semiconductor compounds including a metamorphic layer.Some embodiments of such devices are also known as inverted metamorphicmultijunction solar cells.

2. Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology not only for use in spacebut also for terrestrial solar power applications. Compared to silicon,III-V compound semiconductor multijunction devices have greater energyconversion efficiencies and generally more radiation resistance,although they tend to be more complex to manufacture. Typical commercialcompound semiconductor multijunction solar cells have energyefficiencies that exceed 27% under one sun, air mass 0 (AM0),illumination, whereas even the most efficient silicon technologiesgenerally reach only about 18% efficiency under comparable conditions.Under high solar concentration (e.g., 500×), commercially availableIII-V compound semiconductor multijunction solar cells in terrestrialapplications (at AM1.5D) have energy efficiencies that exceed 44%. Thehigher conversion efficiency of III-V compound semiconductor solar cellscompared to silicon solar cells is in part based on the ability toachieve spectral splitting of the incident radiation through the use ofa plurality of photovoltaic regions with different band gap energies,and accumulating the current from each of the regions.

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as payloads becomemore sophisticated, the power-to-weight ratio of a solar cell becomesincreasingly more important, and there is increasing interest in lighterweight, “thin film” type solar cells having both high efficiency and lowmass.

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures. Theindividual solar cells or wafers are then disposed in horizontal arrays,with the individual solar cells connected together in an electricalseries and/or parallel circuit. The shape and structure of an array, aswell as the number of cells it contains, are determined in part by thedesired output voltage and current.

Inverted metamorphic solar cell structures based on III-V compoundsemiconductor layers, such as described in M. W. Wanlass et al., LatticeMismatched Approaches for High Performance, III-V Photovoltaic EnergyConverters (Conference Proceedings of the 31^(st) IEEE PhotovoltaicSpecialists Conference, Jan. 3-7, 2005, IEEE Press, 2005), present animportant conceptual starting point for the development of futurecommercial high efficiency solar cells. However, the materials andstructures for a number of different layers of the cell proposed anddescribed in such reference present a number of practical difficulties,particularly relating to the most appropriate choice of materials andfabrication steps.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present disclosure provides a methodof manufacturing a solar cell including: providing a first substrate anda second substrate; depositing on the first substrate a sequence oflayers of semiconductor material forming a solar cell including a topsubcell and a bottom subcell; forming a back metal contact over thebottom subcell; forming bonding elements over the surface of the backmetal contact; applying a conductive polyimide adhesive to the secondsubstrate; attaching the second substrate on top of the back metalcontact; and removing the first substrate to expose the surface of thetop subcell.

In another embodiment, the present disclosure provides a method ofmanufacturing a metamorphic multijunction solar cell including:providing a first substrate; depositing on the first substrate asequence of layers of semiconductor material forming a metamorphicmultijunction solar cell including a top subcell and a bottom subcell;forming a back metal contact over the bottom subcell; applying aconductive polyimide adhesive to a second substrate; attaching thesecond substrate on top of the back metal contact; and removing thefirst substrate to expose the surface of the top subcell.

In another embodiment, the present disclosure provides a metamorphicmultijunction solar cell including: a sequence of layers ofsemiconductor material forming a metamorphic multijunction solar cellincluding a top subcell and a bottom subcell; a back metal contactdisposed on the bottom subcell; and a substrate attached to the backmetal contact with a conductive polyimide adhesive.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better and more fully appreciated by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a graph representing the bandgap of certain binary materialsand their lattice constants;

FIG. 2 is a cross-sectional view of the solar cell of the inventionafter the deposition of semiconductor layers on the first substrate;

FIG. 3 is a cross-sectional view of the solar cell of FIG. 2 after thenext process step;

FIG. 4 is a cross-sectional view of the solar cell of FIG. 3 after thenext process step in which a second substrate is attached;

FIG. 5 is a cross-sectional view of the solar cell of FIG. 4 after thenext process step in which the first substrate is removed;

FIG. 6 is another cross-sectional view of the solar cell of FIG. 5 withthe second substrate on the bottom of the Figure;

FIG. 7 is a simplified cross-sectional view of the solar cell of FIG. 6after the next process step;

FIG. 8 is a cross-sectional view of the solar cell of FIG. 7 after thenext process step;

FIG. 9 is a cross-sectional view of the solar cell of FIG. 8 after thenext process step; and

FIG. 10 is a cross-sectional view of the solar cell of FIG. 9 after thenext process step.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

The basic concept of fabricating an inverted metamorphic multijunction(IMM) solar cell is to grow the subcells of the solar cell on asubstrate in a “reverse” sequence. That is, the high band gap subcells(i.e. subcells with band gaps in the range of 1.8 to 2.1 eV), whichwould normally be the “top” subcells facing the solar radiation, areinitially grown epitaxially directly on a semiconductor growthsubstrate, such as for example GaAs or Ge, and such subcells areconsequently lattice-matched to such substrate. One or more lower bandgap middle subcells (i.e. with band gaps in the range of 1.2 to 1.8 eV)can then be grown on the high band gap subcells.

At least one lower subcell is formed over the middle subcell such thatthe at least one lower subcell is substantially lattice-mismatched withrespect to the growth substrate and such that the at least one lowersubcell has a third lower band gap (i.e., a band gap in the range of 0.7to 1.2 eV). A second (e.g., surrogate) substrate or support structure isthen attached or provided over the “bottom” or substantiallylattice-mismatched lower subcell, and the growth semiconductor substrateis subsequently removed. (The growth substrate may then subsequently bere-used for the growth of second and subsequent solar cells).

A variety of different features and aspects of inverted metamorphicmultijunction solar cells are disclosed in the related applicationsnoted above. Some or all of such features may be included in thestructures and processes associated with the solar cells of the presentinvention. Neither, some or all of such aspects may be included in thestructures and processes associated with the semiconductor devicesand/or solar cells of the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

It should be apparent to one skilled in the art, that the inclusion ofadditional semiconductor layers within the cell with similar oradditional functions and properties is also within the scope of thepresent invention.

FIG. 1 is a graph representing the band gap of certain binary materialsand their lattice constants. The band gap and lattice constants ofternary materials are located on the lines drawn between typicalassociated binary materials (such as the ternary material GaAlAs beinglocated between the GaAs and AlAs points on the graph, with the band gapof the ternary material lying between 1.42 eV for GaAs and 2.16 eV forAlAs depending upon the relative amount of the individual constituents).Thus, depending upon the desired band gap, the material constituents ofternary materials can be appropriately selected for growth.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a vapordeposition method, such as Organo Metallic Vapor Phase Epitaxy (OMVPE),Metal Organic Chemical Vapor Deposition (MOCVD), or other vapordeposition methods for the reverse growth may enable the layers in themonolithic semiconductor structure forming the cell to be grown with therequired thickness, elemental composition, dopant concentration andgrading and conductivity type.

FIG. 2 depicts the multijunction solar cell according to the presentinvention after the sequential formation of the three subcells A, B andC on a GaAs first growth substrate. More particularly, there is shown afirst substrate 101, which is preferably gallium arsenide (GaAs), butmay also be germanium (Ge) or other suitable material. For GaAs, thesubstrate is preferably a 15° off-cut substrate, that is to say, itssurface is orientated 15° off the (100) plane towards the (111)A plane,as more fully described in U.S. Patent Application Pub. No. 2009/0229662A1 (Stan et al.). Other alternative growth substrates, such as describedin U.S. Pat. No. 7,785,989 B2 (Sharps et al.), may be used as well.

In the case of a Ge substrate, a nucleation layer (not shown) isdeposited directly on the first substrate 101. On the substrate, or overthe nucleation layer (in the case of a Ge substrate), a buffer layer 102and an etch stop layer 103 are further deposited. In the case of GaAssubstrate, the buffer layer 102 is preferably GaAs. In the case of Gesubstrate, the buffer layer 102 is preferably InGaAs. A contact layer104 of GaAs is then deposited on layer 103, and a window layer 105 ofAlInP is deposited on the contact layer. The subcell A, consisting of ann+ emitter layer 106 and a p-type base layer 107, is then epitaxiallydeposited on the window layer 105. The subcell A is generally latticedmatched to the first growth substrate 101.

It should be noted that the multijunction solar cell structure could beformed by any suitable combination of group III to V elements listed inthe periodic table subject to lattice constant and bandgap requirements,wherein the group III includes boron (B), aluminum (Al), gallium (Ga),indium (In), and thallium (T). The group IV includes carbon (C), silicon(Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N),phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi).

In the preferred embodiment, the emitter layer 106 is composed ofInGa(Al)P and the base layer 107 is composed of InGa(Al)P. The aluminumor Al term in parenthesis in the preceding formula means that Al is anoptional constituent, and in this instance may be used in an amountranging from 0% to 30%.

Subcell A will ultimately become the “top” subcell of the invertedmetamorphic structure after completion of the process steps according tothe present invention to be described hereinafter.

On top of the base layer 107 a back surface field (“BSF”) layer 108preferably p+ AlGaInP is deposited and used to reduce recombinationloss.

The BSF layer 108 drives minority carriers from the region near thebase/BSF interface surface to minimize the effect of recombination loss.In other words, a BSF layer 108 reduces recombination loss at thebackside of the solar subcell A and thereby reduces the recombination inthe base.

On top of the BSF layer 108 is deposited a sequence of heavily dopedp-type and n-type layers 109 a and 109 b that forms a tunnel diode, i.e.an ohmic circuit element that connects subcell A to subcell B. Layer 109a is preferably composed of p++ AlGaAs, and layer 109 b is preferablycomposed of n++ InGaP.

On top of the tunnel diode layers 109 a window layer 110 is deposited,preferably n+ InGaP. The advantage of utilizing InGaP as the materialconstituent of the window layer 110 is that it has an index ofrefraction that closely matches the adjacent emitter layer 111, as morefully described in U.S. Patent Application Pub. No. 2009/0272430 A1(Cornfeld et al.). More generally, the window layer 110 used in thesubcell B operates to reduce the interface recombination loss. It shouldbe apparent to one skilled in the art, that additional layer(s) may beadded or deleted in the cell structure without departing from the scopeof the present invention.

On top of the window layer 110 the layers of subcell B are deposited:the n-type emitter layer 111 and the p-type base layer 112. These layersare preferably composed of InGaP and In_(0.015)GaAs respectively (for aGe substrate or growth template), or InGaP and GaAs respectively (for aGaAs substrate), although any other suitable materials consistent withlattice constant and bandgap requirements may be used as well. Thus,subcell B may be composed of a GaAs, GaInP, GalnAs, GaAsSb, or GaInAsNemitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN base region.

In previously disclosed implementations of an inverted metamorphic solarcell, the middle cell was a homostructure. In some embodiments of thepresent invention, similarly to the structure disclosed in U.S. PatentApplication Pub. No. 2009/0078310 A1 (Stan et al.), the middle subcellbecomes a heterostructure with an InGaP emitter and its window isconverted from InAlP to InGaP. This modification eliminated therefractive index discontinuity at the window/emitter interface of themiddle sub-cell. Moreover, the window layer 110 is preferably dopedthree times that of the emitter 111 to move the Fermi level up closer tothe conduction band and therefore create band bending at thewindow/emitter interface which results in constraining the minoritycarriers to the emitter layer.

In one of the embodiments of the present invention, the middle subcellemitter has a band gap equal to the top subcell emitter, and the bottomsubcell emitter has a band gap greater than the band gap of the base ofthe middle subcell. Therefore, after fabrication of the solar cell, andimplementation and operation, neither the emitters of middle subcell Bnor the bottom subcell C will be exposed to absorbable radiation.Substantially all of the photons representing absorbable radiation willbe absorbed in the bases of cells B and C, which have narrower band gapsthan the respective emitters. In summary, the advantages of theembodiments using heterojunction subcells are: (i) the short wavelengthresponse for both subcells are improved, and (ii) the bulk of theradiation is more effectively absorbed and collected in the narrowerband gap base. The overall effect will be to increase the short circuitcurrent J_(sc).

On top of the cell B is deposited a BSF layer 113 which performs thesame function as the BSF layer 109. The p++/n++ tunnel diode layers 114a and 114 b respectively are deposited over the BSF layer 113, similarto the layers 109 a and 109 b, forming an ohmic circuit element toconnect subcell B to subcell C. The layer 114 a is preferably composedof p++ AlGaAs, and layer 114 b is preferably composed of n++ InGaP.

In some embodiments, barrier layer 115, preferably composed of n-typeInGa(Al)P, is deposited over the tunnel diode 114 a/114 b, to athickness of about 1.0 micron. Such barrier layer is intended to preventthreading dislocations from propagating, either opposite to thedirection of growth into the middle and top subcells A and B, or in thedirection of growth into the bottom subcell C, and is more particularlydescribed in copending U.S. Patent Application Pub. No. 2009/0078309 A1(Cornfeld et al.).

A metamorphic layer (or graded interlayer) 116 is deposited over thebarrier layer 115 using a surfactant. Layer 116 is referred to as agraded interlayer since in some embodiments it is preferably acompositionally step-graded series of InGaAlAs layers, preferably withmonotonically changing lattice constant in each step, so as to achieve agradual transition in lattice constant in the semiconductor structurefrom the lattice constant of subcell B to the lattice constant ofsubcell C while minimizing threading dislocations from occurring. Insome embodiments, the band gap of layer 116 is constant throughout itsthickness, preferably approximately equal to 1.5 eV, or otherwiseconsistent with a value slightly greater than the base bandgap of themiddle subcell B. In some embodiments, the graded interlayer may becomposed of (In_(x)Ga_(1-x))_(y) Al_(1-y)As, with 0<x<1, 0<y<1, and thevalues of x and y selected for each respective layer such that the bandgap of the entire interlayer remains constant at approximately 1.50 eVor other appropriate band gap over its thickness.

In an alternative embodiment where the solar cell has only two subcells,and the “middle” cell B is the uppermost or top subcell in the finalsolar cell, wherein the “top” subcell B would typically have a bandgapof 1.8 to 1.9 eV, then the band gap of the graded interlayer wouldremain constant at 1.9 eV.

In the inverted metamorphic structure described in the Wanlass et al.paper cited above, the metamorphic layer consists of ninecompositionally graded InGaP steps, with each step layer having athickness of 0.25 micron. As a result, each layer of Wanlass et al. hasa different bandgap. In one of the preferred embodiments of the presentinvention, the layer 116 is composed of a plurality of layers ofInGaAlAs, with monotonically changing lattice constant, each layerhaving the same bandgap, approximately 1.5 eV.

The advantage of utilizing a constant bandgap material such as InGaAlAsover a phosphide based material is that arsenide-based semiconductormaterial is much easier to process in standard commercial MOCVDreactors, compared to phosphide materials, while the small amount ofaluminum provides a bandgap that assures radiation transparency of themetamorphic layers.

Although one of the preferred embodiments of the present inventionutilizes a plurality of layers of InGaAlAs for the metamorphic layer 116for reasons of manufacturability and radiation transparency, otherembodiments of the present invention may utilize different materialsystems to achieve a change in lattice constant from subcell B tosubcell C. Thus, the system of Wanlass using compositionally gradedInGaP is a second embodiment of the present invention. Other embodimentsof the present invention may utilize continuously graded, as opposed tostep graded, materials. More generally, the graded interlayer may becomposed of any of the As, P, N, Sb based III—V compound semiconductorssubject to the constraints of having the in-plane lattice parametergreater or equal to that of the second solar cell and less than or equalto that of the third solar cell, and having a bandgap energy greaterthan that of the second solar cell.

Although one embodiment of the present disclosure utilizes a pluralityof layers of AlGaInAs for the metamorphic layer 116 for reasons ofmanufacturability and radiation transparency, other embodiments of thepresent disclosure may utilize different material systems to achieve achange in lattice constant from subcell B to subcell C. Otherembodiments of the present disclosure may utilize continuously graded,as opposed to step graded, materials. More generally, the gradedinterlayer may be composed of any of the As, N, Sb based III-V compoundsemiconductors subject to the constraints of having the in-plane latticeparameter greater or equal to that of the second solar subcell and lessthan or equal to that of the third solar subcell, and having a band gapenergy greater than that of the third solar cell.

In another embodiment of the present invention, an optional secondbarrier layer 117 may be deposited over the InGaAlAs metamorphic layer116. The second barrier layer 117 will typically have a differentcomposition than that of barrier layer 115, and performs essentially thesame function of preventing threading dislocations from propagating. Inthe preferred embodiment, barrier layer 117 is n+ type GaInP.

A window layer 118 preferably composed of n+ type GaInP is thendeposited over the barrier layer 117 (or directly over layer 116, in theabsence of a second barrier layer). This window layer operates to reducethe recombination loss in subcell “C”. It should be apparent to oneskilled in the art that additional layers may be added or deleted in thecell structure without departing from the scope of the presentinvention.

On top of the window layer 118, the layers of subcell C are deposited:the n+ emitter layer 119, and the p-type base layer 120. These layersare preferably composed of n+ type InGaAs and p type InGaAsrespectively, or n+ type InGaP and p type InGaAs for a heterojunctionsubcell, although another suitable materials consistent with latticeconstant and bandgap requirements may be used as well.

A BSF layer 121, preferably composed of InGaAlAs, is then deposited ontop of the cell C, the BSF layer performing the same function as the BSFlayers 108 and 113.

Finally a high band gap contact layer 122, preferably composed ofInGaAlAs, is deposited on the BSF layer 121.

This contact layer added to the bottom (non-illuminated) side of a lowerband gap photovoltaic cell, in a single or a multijunction photovoltaiccell, can be formulated to reduce absorption of the light that passesthrough the cell, so that (1) an ohmic metal contact layer below(non-illuminated side) it will also act as a mirror layer, and (2) thecontact layer doesn't have to be selectively etched off, to preventabsorption.

It should be apparent to one skilled in the art, that additionallayer(s) may be added or deleted in the cell structure without departingfrom the scope of the present invention.

FIG. 3 is a cross-sectional view of the solar cell of FIG. 2 after thenext process step in which a metal contact layer 123 is deposited overthe p+ semiconductor contact layer 122. During subsequent processingsteps, the semiconductor body and its associated metal layers and bondedstructures will go through various heating and cooling processes, whichmay put stress on the surface of the semiconductor body. Accordingly, itis desirable to closely match the coefficient of thermal expansion ofthe associated layers or structures to that of the semiconductor body,while still maintaining appropriate electrical conductivity andstructural properties of the layers or structures. Thus, in someembodiments, the metal contact layer 123 is selected to have acoefficient of thermal expansion (CTE) substantially similar to that ofthe adjacent semiconductor material. In relative terms, the CTE may bewithin a range of 0 to 15 ppm per degree Kelvin different from that ofthe adjacent semiconductor material. In the case of the specificsemiconductor materials described above, in absolute terms, a suitablecoefficient of thermal expansion of layer 123 would range from 5 to 7ppm per degree Kelvin. A variety of metallic compositions and multilayerstructures including the element molybdenum would satisfy such criteria.In some embodiments, the layer 123 would preferably include the sequenceof metal layers Ti/Au/Mo/Ag/Au, Ti/Au/Mo/Ag, or Ti/Mo/Ag, where thethickness ratios of each layer in the sequence are adjusted to minimizethe CTE mismatch to GaAs. Other suitable sequences and materialcompositions may be used in lieu of those disclosed above.

More generally, in other embodiments, the metal contact layer may beselected to have a coefficient of thermal expansion that has a valueless than 15 ppm per degree Kelvin.

In some embodiments, the metal contact layer may have a coefficient ofthermal expansion that has a value within 50% of the coefficient ofthermal expansion of the adjacent semiconductor material.

In some embodiments, the metal contact layer may have a coefficient ofthermal expansion that has a value within 10% of the coefficient ofthermal expansion of the adjacent semiconductor material.

In some embodiments, the metal contact scheme chosen is one that has aplanar interface with the semiconductor, after heat treatment toactivate the ohmic contact. This is done so that (i) a dielectric layerseparating the metal from the semiconductor doesn't have to be depositedand selectively etched in the metal contact areas; and (ii) the contactlayer is specularly reflective over the wavelength range of interest.

FIG. 4 is a cross-sectional view of the solar cell of FIG. 3 after thenext process step in which a second substrate 125 is attached to the topsurface of the metal contact layer 123 using conductive polyimideadhesive bonding layer 124.

In some embodiments, the conductive polyimide adhesive includes apolyimide resin and carbon black particles. The conductive polyimideadhesive can be prepared by mixing an appropriate amount of carbon blackwith a polyimide resin. The amount of carbon black that is added can bevaried such that the cured conductive polyimide adhesive has the desiredresistivity. For some applications, a cured conductive polyimideadhesive having a resistivity of from 100 Ω-cm to 3000 Ω-cm can beutilized. The lower limit is controlled primarily by the adhesiveproperties of the film. The film adhesive strength is inverselyproportional to the amount of carbon black. So one is trading electricalconductivity for adhesion.

A wide variety of polyimide resins can be used including, for example,those available from HD MicroSystems (Partin, N.J.) (e.g., an adhesiveavailable under the trade designation HD-3007 Polyimide Adhesive).

A wide variety of carbon black particles can be used depending on thephysical, chemical, and electrical properties desired. In someembodiments, the carbon black particles have a size of less than orequal to 5 micrometers. Optionally, the conductive polyimide adhesivecan be filtered to remove carbon black particles having a size greaterthan 5 micrometers. Conductive polyimide adhesives having carbon blackparticles less than or equal to 5 micrometers can be especially usefulfor applications that include small features such as bond lines. Theparticle size should be kept below the intended bond line thickness soas to not disrupt the polyimide film.

In some embodiments, the conductive polyimide adhesive includes apolyimide resin and silver flakes. For example, an electricallyconductive polyimide adhesive available from Polymere Technologien(Waldbronn, Germany) under the trade designation POLYTEC EC P-280 isbelieved to contain silver flakes.

In some embodiments, the conductive polyimide adhesive bonding layer 124is deposited over a surface of the second substrate 125. In someembodiments, the conductive polyimide adhesive bonding layer 124 can beapplied to a surface of the second substrate 125 using spin coating toprepare a layer of the desired thickness (e.g., 10 micrometers or less).

In some embodiments, the conductive polyimide adhesive layer on thesecond substrate is cured at a temperature of from 250° C. to 350° C.Optionally and depending on the specific polyimide resin utilized, asoft bake (e.g., 120° C.) of the conductive polyimide adhesive in airmay be utilized prior to curing.

The conductive polyimide adhesive bonding layer 124 is then placedadjacent to the metal contact layer 123, so that the second substrate125 is bonded to and adheres to the semiconductor structure. Forexample, a conventional wafer bonder can be used with appropriatebonding conditions (e.g., 5 kN at 300° C. for 30 minutes).

In some embodiments, the second substrate 125 is a flexible substratesuch as a metallic flexible film. In some embodiments, the metallicflexible film has a thickness of approximately 50 microns, or moregenerally, between 0.001 and 0.01 inches. An alternative substrateimplementation would be 0.002″ Kapton film plus 0.0015″ adhesive/0.002″Mo Foil/0.002″ Kapton film plus 0.0015″ adhesive for a total thicknessof 0.009″. However the Kapton film can be as thin as 0.001″ and as thickas 0.01″. The adhesive can be as thin as 0.0005″ and as thick as 0.005″.The Mo foil can be as thin as 0.001″ and as thick as 0.005″. In someembodiments, the metallic flexible film includes a molybdenum layer

In some embodiments, the second substrate 125 is a rigid substrate. Insome embodiments, the second substrate 125 is glass, as described inU.S. patent application Ser. No. 13/547,334 filed Jul. 12, 2012 notedabove. Alternatively, the second substrate 125 may be sapphire, GaAs, Geor Si, or other suitable material. In such alternative embodiments, thesecond substrate 125 may range from 4 mils to 10 mils.

FIG. 5 is a cross-sectional view of the solar cell of FIG. 4 after thenext process step in which the first growth substrate 101 is removed. Insome embodiments, the first substrate 101 may be removed by a sequenceof lapping, grinding and/or etching steps in which the first substrate101, and the buffer layer 103 are removed. The choice of a particularetchant is growth substrate dependent. In other embodiments, the firstsubstrate may be removed by a lift-off process such as described in U.S.Patent Application Pub. No. 2010/0203730 A1 (Cornfeld et al.), herebyincorporated by reference.

FIG. 6 is a cross-sectional view of the solar cell of FIG. 5 with theorientation with the second substrate 125 being at the bottom of theFigure. Subsequent Figures in this application will assume suchorientation.

FIG. 7 is a simplified cross-sectional view of the solar cell of FIG. 6depicting just a few of the top layers and lower layers over the secondsubstrate 125, with the orientation with the second substrate 125 beingat the bottom of the Figure. Subsequent Figures in this application willassume such orientation.

FIG. 8 is a cross-sectional view of the solar cell of FIG. 7 after thenext process step in which the etch stop layer 103 is removed by aHCl/H₂O solution.

FIG. 9 is a cross-sectional view of the solar cell of FIG. 8 after thenext sequence of process steps in which a photoresist mask (not shown)is placed over the contact layer 104 to form the grid lines 501. As willbe described in greater detail below, the grid lines 501 can bedeposited via evaporation and lithographically patterned and depositedover the contact layer 104. The mask can be subsequently lifted off toform the finished metal grid lines 501 as depicted in the Figures.

As more fully described in U.S. Patent Application Pub. No. 2010/0012175A1 (Varghese et al.), hereby incorporated by reference, the grid lines501 are preferably composed of the sequence of layers Pd/Ge/Ti/Pd/Au,although other suitable sequences and materials may be used as well.

FIG. 10 is a cross-sectional view of the solar cell of FIG. 9 after thenext process step in which the grid lines are used as a mask to etchdown the surface to the window layer 105 using a citric acid/peroxideetching mixture.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofconstructions differing from the types of constructions described above.

Although some of the embodiments of the present invention utilizes avertical stack of three subcells, the present invention can apply tostacks with fewer or greater number of subcells, i.e. two junctioncells, four junction cells, five junction cells, etc. as moreparticularly described in U.S. Pat. No. 8,236,600 (Cornfeld). In thecase of four or more junction cells, the use of more than onemetamorphic grading interlayer may also be utilized, as moreparticularly described in U.S. Patent Application Pub. No. 2010/0122724A1 (Cornfeld et al.).

In addition, although in some embodiments the solar cell is configuredwith top and bottom electrical contacts, the subcells may alternativelybe contacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, embodiments of the present invention may utilize anarrangement of one or more, or all, homojunction cells or subcells,i.e., a cell or subcell in which the p-n junction is formed between ap-type semiconductor and an n-type semiconductor both of which have thesame chemical composition and the same band gap, differing only in thedopant species and types, and one or more heterojunction cells orsubcells. Subcell A, with p-type and n-type InGaP is one example of ahomojunction subcell. Alternatively, as more particularly described inU.S. Patent Application Pub. No. 2009/0078310 A1 (Stan et al.), thepresent invention may utilize one or more, or all, heterojunction cellsor subcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor havingdifferent chemical compositions of the semiconductor material in then-type regions, and/or different band gap energies in the p-typeregions, in addition to utilizing different dopant species and type inthe p-type and n-type regions that form the p-n junction.

In some embodiments, a thin so-called “intrinsic layer” may be placedbetween the emitter layer and base layer of some subcells, with the sameor different composition from either the emitter or the base layer. Theintrinsic layer may function to suppress minority-carrier recombinationin the space-charge region by minimizing interdiffusion of the n-typeand p-type dopants on either side of the junction. Similarly, either thebase layer or the emitter layer may also be intrinsic ornot-intentionally-doped (“NID”) over part or all of its thickness. Somesuch configurations are more particularly described in U.S. PatentApplication Pub. No. 2009/0272438 A1 (Cornfeld).

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and in some embodiments may include AlInP, AlAs, AlP,AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs,AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb,AlN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, andsimilar materials, and still fall within the spirit of the presentinvention.

Although the invention has been illustrated and described as embodied inan inverted metamorphic multijunction solar cell, it is not intended tobe limited to the details shown, since various modifications andstructural changes may be made without departing in any way from thespirit of the present invention.

Thus, while the description of this invention has focused primarily onsolar cells or photovoltaic devices, persons skilled in the art knowthat other optoelectronic devices, such as, thermophotovoltaic (TPV)cells, photodetectors and light-emitting diodes (LEDs) are very similarin structure, physics, and materials to photovoltaic devices with someminor variations in doping and the minority carrier lifetime. Forexample, photodetectors can be the same materials and structures as thephotovoltaic devices described above, but perhaps more lightly-doped forsensitivity rather than power production. On the other hand LEDs canalso be made with similar structures and materials, but perhaps moreheavily-doped to shorten recombination time, thus radiative lifetime toproduce light instead of power. Therefore, this invention also appliesto photodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

The invention claimed is:
 1. An inverted metamorphic multijunction solarcell comprising: a sequence of layers of semiconductor material formingan inverted metamorphic multijunction solar cell including a top subcelland a bottom subcell; a back metal contact disposed on the bottomsubcell; and an electrically conductive substrate attached to the backmetal contact with a cured electrically conductive polyimide adhesive,wherein the electrically conductive substrate comprises a metallicflexible film comprising a molybdenum layer.
 2. The solar cell of claim1, wherein the electrically conductive polyimide adhesive comprisescarbon black particles.
 3. The solar cell of claim 2, wherein the carbonblack particles have a size of less than or equal to 5 micrometers. 4.The solar cell of claim 1, wherein the electrically conductive polyimideadhesive comprises silver flakes.
 5. The solar cell of claim 1, whereinthe cured electrically conductive polyimide adhesive has a resistivityfrom 100 Ω-cm to 3000 Ω-cm.
 6. The solar cell of claim 1, wherein theelectrically conductive substrate is a flexible substrate.
 7. A solarcell comprising: a sequence of layers of semiconductor material forminga solar cell including a top subcell and a bottom subcell; a back metalcontact disposed on the bottom subcell; and an electrically conductivesubstrate attached to the back metal contact with a cured electricallyconductive polyimide adhesive, wherein the electrically conductivesubstrate comprises a metallic flexible film comprising a molybdenumlayer.
 8. The solar cell of claim 7, wherein the electrically conductivepolyimide adhesive comprises carbon black particles.
 9. The solar cellof claim 8, wherein the carbon black particles have a size of less thanor equal to 5 micrometers.
 10. The solar cell of claim 7, wherein theelectrically conductive polyimide adhesive comprises silver flakes. 11.The solar cell of claim 7, wherein the cured electrically conductivepolyimide adhesive has a resistivity from 100 Ω-cm to 3000 Ω-cm.
 12. Asolar cell comprising: a sequence of layers of semiconductor materialforming a solar cell including a top subcell and a bottom subcell; aback metal contact disposed on the bottom subcell; and an electricallyconductive substrate attached to the back metal contact with anelectrically conductive polyimide adhesive, wherein the back metalcontact comprises molybdenum.
 13. The solar cell of claim 12 wherein theback metal contact comprises a sequence of metal layers selected fromthe group consisting of Ti/Au/Mo/Ag/Au, Ti/Au/Mo/Ag, and Ti/Mo/Ag. 14.An inverted metamorphic multijunction solar cell comprising: a sequenceof layers of semiconductor material forming an inverted metamorphicmultijunction solar cell including a top subcell and a bottom subcell; aback metal contact disposed on the bottom subcell, wherein the backmetal contact comprises a sequence of metal layers selected from thegroup consisting of Ti/Au/Mo/Ag/Au, Ti/Au/Mo/Ag, and Ti/Mo/Ag; and anelectrically conductive substrate comprising a metallic flexible filmcomprising molybdenum attached to the back metal contact with a curedelectrically conductive polyimide adhesive comprising silver flakesand/or carbon black particles having a size of less than or equal to 5micrometers, wherein the cured electrically conductive polyimideadhesive has a resistivity from 100 Ω-cm to 3000 Ω-cm.